Study of Alfvén wave reflection to address the solar coronal heating problem*

The physics behind the heating of the solar corona and the acceleration of the fast solar wind from coronal holes (predominantly open field regions of the solar corona) is not well understood. Recent observations of large-amplitude counter-propagating Alfvén waves at the base of coronal holes suggest that the outward and the inward waves interact nonlinearly to generate turbulence which heats and accelerates the plasma. However, the mechanism of generating inward Alfvén waves is yet to be fully established. Most theories within the MHD framework invoke partial reflection of outward Alfvén waves from gradients in the Alfvén speed to explain the inward waves. To date, though, no experiment has reported the detection of a reflected Alfvén wave in an experimental arrangement relevant to coronal holes. We have performed new experiments to detect a reflected wave from an Alfvén speed gradient under conditions scaled to match coronal holes. The experiments were conducted in the Large Plasma Device at the University of California, Los Angeles. Our results show that the reflected Alfvén wave energy increases as the ratio of the wavelength to the gradient scale length increases. The results of the experiments are presented.